BIVALVIA. 113 



From Mactra gibbosa, Sow., this species is distinguished by the less elevated and 

 compressed umbones, by the more straight and horizontal superior and inferior borders, 

 and more especially by the very marked depression of the anterior side, which in M. 

 yibbosa is convex. The test upon the anterior side and near to the umbones has a 

 considerable degree of thickness ; the rib of the left valve deeply indents the cast. The 

 surface of the test, although identical in character with that of other species of Myacites, 

 has never exhibited any distinct portion of granulated surface ; had the granules been large, 

 they could scarcely have failed to have been preserved equally with the surfaces of other 

 species in the same bed. Indications of a few radiating lines, near to the umbones, are 

 sometimes obscurely visible upon the test. 



Dimensions of a small Great Oolite specimen. Height, \\ inch, length, 2^ inches, 

 diameter through both the valves, 1 inch ; but the examples from the Inferior Oolite of the 

 Cotteswolds are not uncommonly more than double these dimensions. 



Geological position and localities. Ponton, Lincolnshire, in the Great Oolite ; 

 Rodboro'-hill, near Stroud, in the gryphite grit of the Inferior Oolite; at the latter 

 locality the test is preserved; it also occurs not uncommonly in the form of casts 

 throughout the Cotteswolds, in the same stratum ; but it has usually been confounded with 

 Mactra gibbnw. Sow., a shell whose test is rarely preserved, and which does not occur so 

 low as the gryphite grit. 



Myacites calceiformis, Phil. sp. Tab. XI, fig. 2. 



Mya margaritifera, Young and Bird. Geol. York. Coast, pi. 7, fig. 2. 

 Mya calceiformis, Phil. Geol. York., 1, t. 11, fig. 3. 



Testa elongatd, compressd, antice subconvexd, postice compressd et attenuatd, costd 

 unicd obscurd ab umbone ad basin instructd, lateribus plicis longitudinalibus irregularibus, 

 testa delicatissima, granulis radiantibus crebris minutis. 



Shell elongated, somewhat compressed ; umbones acute, anterior to the middle of the 

 valves ; anterior side rather convex, its margin rounded, and the aperture narrow ; posterior 

 side compressed, lengthened, and somewhat pointed, its aperture small ; superior margin 

 sloping obliquely, nearly straight ; inferior margin nearly straight ; a single obscure 

 elevation extends from the umbo to the inferior border, and there is, occasionally, posterior 

 to it, a wide superficial depression ; the longitudinal plications are numerous, fine, and 

 irregular ; the test is of extreme tenuity, and covered with lines of very minute radiating 

 granules. 



Specimens are usually destitute of the delicate test, but well preserved portions of it 

 are occasionally found. It is nearly allied to Mgopsis marginata, Ag., 'Etud. Crit. Moll.,' 

 tab. 30, fig. 1, 2, but the species of Agassiz has a shorter anterior side, less rounded, and 

 the convexity of the valves is more considerable ; it is also higher and shorter than the 



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